Low viscosity liquid bridges: Stretching of liquid bridges immersed in a higher viscosity liquid

We examine the dynamics of a liquid bridge between a sphere and a flat plate being separated from each other. Unlike previous research, this paper focuses on the case where the viscosity of the bridge is lower than that of the external fluid within which the particle, the plate, and the liquid bridge are immersed. For the general case of a viscosity mismatch between the bridge fluid and the external fluid, we develop a lubrication theory-based model for the viscous force during separation. The model predicts that a low viscosity bridge reduces the force as compared to both - separation without a liquid bridge, or separation with a bridge of matched viscosity. The magnitude of force reduction is expected to be more severe at small sphere-plate separations and at large bridge volumes. Experiments confirm all these predictions qualitatively, but unexpectedly the magnitude of the reduction is even larger than predicted. Experiments also find that the bridge length at rupture for specified velocity exceeds that for quasistatic rupture by an amount that increases with the squareroot of the velocity. Although we only examine bridges between a plate and a spherical particle, all results are expected to apply for bridges between a pair of particles as well.